How many Drupal sites are there in the world?

 How do you find "all" the Drupal sites that Google knows about?

A post on suggests Add New Comment, but that wont be entirely accurate. So far I have come up with a search that lists all sites with a User Account page on user/login:

intitle:"User account" inurl:"user/login"

Or you can use this link to search for drupal sites in Google.

I am aware this only lists sites with user login linked to somewhere and not blocked by Robots.txt.

I have also seen suggestions that looking for a CSS file might work, however with Drupal's CSS aggregation, this could be difficult.

Please share any alternatives, my search above (currently) shows around 496,000 (on Google UK). How can we make this better?